It’s how both platforms (and I assume the rest of fedi) work. If you follow an account from Mastodon, that your server has never seen before, it’ll be blank until new posts are made. That goes for other Mastodon accounts, Lemmy communities / users, Pixelfed accounts, PeerTube channels, etc etc.
If you subscribe to a Lemmy community, and your Lemmy server has never seen that community before, same thing. Blank until new posts.
It caused a lot of confusion and tech support last summer but now that most servers know about most other ones I guess it’s not as common to run into these days :D